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This is likely due to the video codec that you are using. First, an explanation of terms is required: WMV is a container format. These can contain one or more video or audio streams - with video this is normally one video & one audio stream; and these are encoded with various codecs (coder/decoder). I'm not very familiar with the WMV format - for editing I use proper intermediate codecs (Apple ProRes in my case; more in this in a moment), and I prefer MP4 as a delivery codec.

There are broadly three 'types' of video codec: capture codecs, that a video camera will use to record the video; intermediate codecs, that are more suited to post-production editing (they trade larger files for requiring less processing power & the ability to easily cut on any frame); and delivery codecs, that are more compressed (so they can be streamed over the internet, or so you can fit a decent length on a physical medium like a DVD, or for broadcast). There is some overlap between these (many DSLRs use h.264, which is designed to be a delivery codec, to capture video).

This stuff also applies to audio - though there you should use uncompressed PCM audio. However, since audio decompression requires far less processing power, some NLEs will allow you to get away with compressed audio.

I have never heard of the WMV format containing any intermediate formats, as far as I know it's a purely delivery format. That's probably the cause of your problem. I've never used Avid, so I don't know exactly which intermediate codec you should use. Since it's designed by Avid, the DNxHD codec is a good bet for video (you should convert your audio to PCM audio while you're at it).

Your video editing software probably came with a media conversion tool. You could also use the command-line-based ffmpeg (wiki) to encode to DNxHD. For the following command, you would need to adjust the bit rate - here it is 36 megabits/second - see here for some information on what you should use. Note that the ffmpeg syntax used in that guide is somehat out of date; you should use -c:v instead of -vcodec and -b:v instead of -b. This following command will also convert the audio to uncompressed 16-bit PCM, for maximum compatibility: